Nanodots to launch large memories

PORTLAND, Ore.  Arrays of 7-nanometer magnetic nickel nanodots, assembled by researchers at the National Science Foundation's Center for Advanced Materials and Smart Structures (CAMSS), aim at a 500x increase in memory density, to 10 trillion bits per square inch.

At such densities, coin-sized chips holding 5 terabits each could pack the entire Library of Congress into "a pocket full of change," said Jagdish Narayan, professor of materials science and engineering at North Carolina State University and CAMSS director at the university. Narayan performed the work with research associate Ashutosh Tiwari.

The technique, which uses pulsed-laser ablation to make nanodots that are said to be 10 times smaller than previously possible, can also be used to make more-efficient LEDs, single-electron transistors, spin transistors, hybrid devices, superhard coatings and novel biomaterials.

The key to the technique, according to the researchers, is its ability to precisely control the assembly of nanodots in three dimensions. Most conventional self-assembly techniques for integrated circuitry involve only two-dimensional planar operations, which are performed in sequence to build up the 3-D layers of a microchip.

"To make nanostructures this tiny, self-assembly in three dimensions is essential," said Narayan. "Before, we could only make single-layer structures; but by carefully controlling the kinetics, we can now self-assemble in three dimensions."

The technique uses pulsed-laser molecular-beam epitaxy to precisely deposit the matrix in which the metal nickel nanodots will self-assemble into arrays. As a result, instead of the typical distribution of nanodots of different sizes, the nanodots produced with the (CAMSS) process all measure 7 nm across, or a tenth the diameter of existing nanodots, according to Narayan.

The technique works by reducing the number of imperfections, which ordinarily are rife at such small sizes. "Nature tries to create nanodots in a distribution of different sizes to reduce the energy in the matrix"it doesn't like uniform sizes, because they have a higher energy state," said Narayan. "But with our technique you can efficiently and reliably create nanostructures using the same size nano-units."

The technique was applied experimentally to embed the 7-nm dots into ordered 3-D arrays within a matrix of aluminum oxide and titanium nitride atoms. Since each dot can have its magnetic spin state either up or down, each 7-nm magnetic domain can store a single bit. Thus, such materials could someday be used to create 1-inch2 chips that would hold 10 Tbits of data.

According to the researchers, many processes will benefit from the technique's increased control and precision. To adapt the approach for production of more-efficient LEDs, the researchers are experimenting with using similar steps in the self-assembly of light-emitting gallium indium nitride nanodots in a matrix of zinc oxide and gallium nitride atoms.

Currently the researchers are working with Kopin Corp. (Taunton, Mass.) to reduce the number of imperfections in Kopin's CyberLite LEDs by employing a variation of Narayan's technique that Kopin has dubbed NanoPockets.
"What we do to increase the efficiency of LEDs," Narayan said, "is reduce the imperfections by making a beady layer of nanodots that confines the carriers in a perfect matrix."

Base of discovery

According to the National Science Foundation, the infrastructure of nanotechnology research is creating a huge base of scientific discovery for technological development. For instance, in 1999 there were only five universities with graduate programs in nanotechnology, but in 2004 there are 270 academic undergraduate and graduate programs related to nanoscale science and engineering, according to NSF.

"Narayan's work achieves a general objective of the United States' National Nanotechnology Initiative: the systematic control of nanoscale features in order to obtain new properties and functions," said Mihail Roco, NSF senior adviser for nanotechnology. "By using self-assembly to create a 3-D array of nanodots, Narayan's technique may have significant applications in lighting, lasers, spintronics and optical devices.

"In the next two to three years, practical applications like nanodot lighting systems may have significant environmental, economic and energy-saving advantages," Roco said.

The National Science Foundation supports fundamental research with an annual budget of nearly $5.58 billion, offering grants to nearly 2,000 universities and institutions. NSF receives about 40,000 requests for funding each year, and makes about 11,000 new funding awards each year.